Despite successful immunization of children against 7 serotypes of S. pneumoniae (SP) with pneumococcal conjugate vaccine (PCV), critical questions remain regarding long-term vaccine effects on SP serotypes and antimicrobial resistance. Replacement with non-vaccine types (through expansion of already circulating strains, importation of new strains, or capsular switching) and emerging antibiotic resistance could again increase morbidity from pneumococcal infection. Building on our analysis of SP isolates from 16 communities in 2001 and 2004, we will perform a multi-community assessment of changes in carriage prevalence and serotype-specific antimicrobial resistance in 2007 and 2009. Isolates will undergo resistance testing, serotyping, pulse field gel electrophoresis, and multi-locus sequence typing to determine the biological processes which underlie changes in community carriage of SP in the era of universal immunization. Specifically we will: 1)Measure trends in carriage of SP, including prevalence, serotypes carried, and serotype-specific antibiotic resistance. 2) Test competing hypotheses for shifts in pneumococcal population structure following PCV7 introduction, including the mechanism of antibiotic resistance among non-vaccine types. 3) Determine if previously documented risk factors for carriage of SP (susceptible and non-susceptible) continue to predict carriage in the post-PCV7 era. Continuing assessment of the epidemiology and population genetics of the community reservoir of SP will inform future expectations regarding localized and invasive pneumococcal disease.